1. Field of the Invention
This invention relates to a method and an apparatus for validating banknotes.
2. Background
It is well known to validate banknotes by taking measurements of the optical characteristics of the banknote and to process the measurements together with acceptance criteria to determine whether the banknote belongs to a predetermined class, or denomination. The banknote can be scanned and reflected or transmitted light, or both, can be used to measure the optical characteristics. The characteristics of the banknote at different light wave lengths (some or all of which may be non-visible) may be measured.
The characteristics of the components of the apparatus, such as light emitters or sensors, may vary from apparatus to apparatus, and from time to time. Consequently, the sensors of the apparatus cannot be relied upon to give stable and predictable measurements.
It is known to mitigate this problem by frequently calibrating the apparatus. Various calibration techniques can be used. For example, in reflective systems, a reflective surface may be provided on the opposite side of the bill path from the light emitter and sensor so that, when no bill is present, a calibration measurement may be made by illuminating the surface and detecting the amount of light reflected to the sensor. This calibration measurement may be used to adjust the intensity of the light emitted by the emitter and/or the gain applied to the signal from the sensor so that a predetermined measurement is obtained.
This technique is not readily adaptable to systems in which light transmission through the banknote is measured, because the reference surface would interfere with the light path. One solution to this, involving a movable reference surface, is disclosed in EP-A-0731737. Alternatively, the reference surface may take the form of a calibration sheet which is moved into the bill path when a calibration measurement is to be performed.
EP-A-0679279 discloses an apparatus for detecting counterfeit banknotes in which the banknote is manually swept past a light emitter and sensor housed within a unit having a glass window. In that arrangement, the lamp intensity is monitored by detecting the amount of radiation internally reflected from the window. However, such an arrangement is also unsuitable for transmission systems.
These calibration techniques also suffer from a number of further disadvantages. For example, when using a calibration sheet, the calibration operation either needs to be performed manually, which is inconvenient and inappropriate where frequent calibration is required, or, if it is automatic, complicated sheet-driving structures need to be provided. Also, the calibration techniques rely upon the reference surfaces having stable optical characteristics, and this is not always the case; for example, the optical characteristics may alter as a result of contamination by dirt, etc.
It is also known to mitigate the problem of component variation by performing normalisation of sensor measurements. See for example EP-A-0560023. Each banknote is scanned along respective different tracks. Within each track, for each colour being measured, the same components are used to take the measurements throughout the track. The measurements are normalised by taking the ratio of the measurement to the sum of the measurements for the same colour throughout a scanned track along the banknote (“spatial normalisation”). Therefore, the effects of component variation are reduced.
However, such spatially normalised measurements are relatively insensitive to the relative amounts of different colours and therefore are not suitable for accurate authentication of the banknotes. Accordingly, measurements were also normalised using another technique. According to this further technique, the measurements of different colours within a particular region were normalised by deriving the ratio of each measurement to the sum of all the measurements for the different colours in that area. This “spectral normalisation” technique maintains the colour information and thus is useful for authentication. Also, the technique effectively makes the measurements insensitive to the brightness in each region of the banknote, and thus less sensitive to the amount of dirt on the banknote. Accordingly, new (clean) and old (dirty) banknotes will exhibit less measurement dispersion, and thus improve recognition performance. However, because the spectrally normalised measurements are sensitive to component variations, and because the luminance information is reduced, the measurements are not good for determining banknote denomination.
Accordingly, although current banknote validators deal with the problem of component variation, both the normalisation techniques and the calibration techniques would benefit from improvements, particularly (but not only) in transmission systems.